1. Field of the Invention
The present invention relates to lamps and other illumination devices, and in particular relates to LDE-based lamps using a novel Fresnel lens.
2. Description of the Related Art
A typical machine vision system consists of an illuminator, a camera, and a control system for enabling the illuminator and camera in combination to capture an image of an object which is under inspection and for processing the image and initiating a desired action. The action may be marking the product as acceptable or rejected. The illuminator is an important part of the system, and properties of illumination such as brightness, uniformity, angle of incidence, color, and degree of collimation can have a significant influence on the overall performance of the vision system. Additionally, the physical size of the illuminator is very important, as for some vision systems, there is very little physical space available for the illuminator. Several types of illuminators have been used within machine vision systems. For example, types of illuminators include incandescent bulbs, fluorescent tubes, lasers, Xenon flash tubes, halogen bulbs combined with fibre light guides, and light emitting diodes (LEDs). Of the illuminators, LEDs have become increasingly popular due to their relative lower costs, smaller sizes, longer operating lifespan, faster switching speeds and efficiency.
Augustin Fresnel, for whom the Fresnel lens is named, constructed a lens in which the center of the curvature of different rings recedes from an axis according to center distances, so as to practically eliminate spherical aberration. Fresnel lenses are used as light condensers, illuminators, and magnifiers, among other ways. Fresnel lenses basically consist of a series of concentric prismatic grooves, designed to cooperatively direct incident light rays to a common focus. This type of lens is thin, lightweight, and of high aperture.
Generally, for prior art thin Fresnel lenses used for visible light applications, the grooves are all the same width, so that the groove density is constant across the lens. Meanwhile, conventionally, a lens-type LED package well-known in the art, uses a light emitting element as a light source and is designed to radiate light emitted from the light emitting element in a predetermined direction while controlling its lighting distribution by using an optical system.
However, in the lens-type LED package, it is impossible to secure a same angle of the lens surface to the light emitting element when it is provided with an optical surface shape to enhance the convergence performance. Therefore, high efficiency of external radiation cannot be achieved as only about 30% of light emitted from the light emitting element can be controlled.
FIGS. 1A and 1B are cross-sectional views illustrating a structure of an LED package using a conventional Fresnel lens. As shown in FIG. 1A, an LED package 10 is composed of an LED 12 as a light source and a lens 14 to converge radiating light emitted from the LED 12. As shown in FIG. 1B, the lens 14 includes facet surfaces 142 and draft surfaces 144, and the draft surfaces has a vertical angle (90°) which are perpendicular to the horizontal surface. Generally, when the light L emitted form LED 12 passes through the facet surfaces 142, only some portions of the light are collimated to a substantially parallel light so that it is difficult to collimate all portions of the light effectively. Further, if the light L passes through the draft surface 144, the light cannot be collimated and becomes a scattered light, which is normally called draft loss. Additionally, the conventional structure of the lens 14 is only designed for a point light source, so poor collimation effect is seen for an area light source.
Thus, a novel method and a Fresnel lens structure are required to mitigate the previously mentioned problems.